Devices for picking up and discharging liquid, for example blood, employing a capillary tube are well-known to the art. U.S. Pat. Nos. 3,828,987 and 3,720,354 each disclose a capillary tube containing a piston which is withdrawn to pick up liquid and advanced to discharge liquid. The presence of the piston in the capillary tube while liquid is being drawn into the tube is a disadvantage when consistent accuracy is desired since it can cause the presence of an air bubble in the capillary tube. While the prior art recognizes the desirability of having such devices disposable, it still contemplates using operating parts to be removed from the disposable capillary tube and piston for reuse. This involves time-consuming operations. These problems are solved in this invention by providing a capillary tube mounted on a support member in alignment with a piston with space for the passage of air from the capillary tube as it picks up liquid. The entire unit can be disposed of after use.